PS3 Blu-Ray issues

First of all, sorry if this has already been dealt with elsewhere on the forum.

I'm having an odd problem with my PS3, which I'm running through a component cable (HDMI ports are busted on my 4-year-old Bravia). On Monday I played a Frozen Planet Blu Ray, and it displayed gorgeously in HD. Yesterday I put the same disc back in the PS3 to watch another episode and saw that the PS3 switched to 576p, which annoyed me immensely (I refused to watch the series on TV with the wife because I specifically wanted to watch it in HD!).

I'm sure this has happened occasionally with Blu Ray playback before, and seems to be sporadic, regardless of updates, etc.